When an air bridge is formed on a silicon substrate, generally a method of forming an air bridge by making use of difference in etching rate for each crystal plane of silicon has been employed. The difference in etching rate for each crystal plane of silicon is the phenomenon that, when a single crystal of silicon is etched with a particular etching liquid (EPW liquid), the etching rate largely varies for each crystal axis and the etching rate in a certain direction (111 direction) is extremely lower than those in other directions (100 direction and 110 direction). As shown in FIGS. 18A and 18B, for instance, when a mask layer 1802 is formed on a (100) plane of a silicon substrate 1801 and the mask layer 1802 is etched with an etching liquid, a cavity surrounded by four (111) planes is formed. In other words, etching proceeds until all planes of the silicon substrate 1801 being exposed to the etching liquid become (111) planes (Refer to FIGS. 18C and 18D). This type of etching is called anisotropic etching.
In the method of forming an air bridge by making use of difference in etching rate for each crystal plane of silicon, an air bridge 1803 is formed, as shown in FIG. 19B or FIG. 20B, by forming the mask layer 1802 on the silicon substrate 1801 having an air bridge pattern as shown in FIG. 19A or FIG. 20A and executing anisotropic etching.
With the conventional type of etching method for silicon substrate, however, although air bridges such as those shown in FIGS. 19 and 20 can be formed, as there are various technological restrictions concerning an air bridge pattern along which an air bridge is formed because of characteristics of anisotropic etching, there have been such problems as limited availability for various purposes and a small freedom in pattern layout on a silicon substrate.
For instance, even if an air bridge pattern 2101 (a portion indicated by dotted lines having a large area as shown in FIG. 21A) is formed, anisotropic etching is completed in the states as shown in FIGS. 21B and 21C, so that an air bridge can not be formed.
On the other hand, the air bridge 1803 can be made larger by employing the cantilever form, but in this case the strength and the stability of the air bridge are rather low. Also the air bridge 1803 can be made larger by forming it in approximately 45 degrees diagonally inclined state, but also in this case the strength and the stability of the air bridge are low. In either case, the freedom of layout on a silicon substrate including an air bridge pattern is extremely low.